1. Field of the Invention:
The present invention relates to a printing-plate preparation apparatus employed in a screen printing machine in which a stencil paper is clamped between a thermal printhead and a platen roller so as to be delivered while perforated by means of a heating-element portion of the thermal printhead, and more particularly to the printing-plate preparation apparatus in which the platen roller is improved.
2. Description of the Prior Art:
The platen roller employed in the printing-plate preparation apparatus employed in the screen printing machine is provided with an outer peripheral layer made of a conventional synthetic rubber such as chloroprene rubber, nitrile rubber and the like.
As shown in FIG. 1A, in case that the outer peripheral layer 101a of the platen roller 101 is made of the above-mentioned conventional synthetic rubber, parts of a film layer 118 and an adhesive layer 119 of the stencil paper 105 are melted to pass through a base layer 117 of the stencil paper 105 and then adhere to the outer peripheral surface 101a of the platen roller 101 when the thermal printhead 102 conducts a perforation operation of the stencil paper 105, so that a deposit 120 of such parts of the stencil paper 105 is formed on the outer peripheral surface of the platen roller 101 each time the perforation operation of the stencil paper 105 is conducted by the thermal printhead 102, whereby the amount of such deposit 120 increases as the number of such perforation operations increases. The deposit 120 causes the base layer 117 of the stencil paper 105 to adhere to the outer peripheral layer 101a of the platen roller 101. As a result, as shown in FIG. 1B, the platen roller 101 is eventually wrapped around with the stencil paper 105 to make it impossible to deliver the printing plate or perforated stencil paper 105. This is a defect inherent in the conventional printing-plate preparation apparatus.
The thermal printhead 102 is fixedly mounted on an end portion of a bracket 103 which is swingably supported by its central axle 104 as shown in FIG. 1B. The other end portion of the bracket 103 is engaged with a machine frame 106 through a tension spring 107 for urging the thermal printhead 102 against the platen roller 101.
In a stencil paper discharging side of the platen roller 101 and the thermal printhead 102, there are provided a pair of spaced inclined guide plates 108 adjacent to which are provided a feed roller 109 and a pressure roller 110. A supporting portion 111 for the pressure roller 110 is mounted on a compression spring 112 for urging the pressure roller 110 against the feed roller 109.
On the other hand, in case that a width of the stencil paper 105 is reduced, for example, the size of the stencil paper 105 is changed from a size "B" sheet (not shown) to a size "A" (not shown) which is smaller in width than the size "B" sheet, since opposite end portions of the platen roller 101 extend outward from opposite edges of the size "A" stencil paper 105, the platen roller 101 is brought into contact with a surface of the thermal printhead 102 at its opposite end portions of the outer peripheral layer 101a. In this case, however, in the conventional printing-plate preparation apparatus, since the outer peripheral layer 101a of the platen roller 101 is made of a frictional material as described above, a large frictional resistance is produced when the platen roller 101 is brought into contact with the surface of the thermal printhead 102, so that the motor (not shown) for delivering the perforated stencil paper 105 or printing plate is overloaded. This is another defect inherent in the conventional printing-plate preparation apparatus.